Use the routing id on api::WebContents::GetID

The sandbox option allows multiple webContents in one renderer process, so using
the only the renderer id to identify WebContents instances is no longer an
option.

WebContents::GetID now returns a 64-bit integer, which is composed of both the
process id(high 32), and the RenderViewHost routing id(low 32). Also add a
`GetProcessID` that retrieves the renderer process id, a requirement in some of
our javascript code.

atom/browser/api/atom_api_web_contents.h

@@ -68,7 +68,8 @@ class WebContents : public mate::TrackableObject<WebContents>,
   static void BuildPrototype(v8::Isolate* isolate,
                              v8::Local<v8::FunctionTemplate> prototype);
 
-  int GetID() const;
+  int64_t GetID() const;
+  int GetProcessID() const;
   Type GetType() const;
   bool Equal(const WebContents* web_contents) const;
   void LoadURL(const GURL& url, const mate::Dictionary& options);